Abstract

We map a nanoindent in a silicon carbide (SiC) crystal by infrared (IR) scattering-type scanning near-field optical microscopy (s-SNOM) and confocal Raman microscopy and interpret the resulting images in terms of local residual stress-fields. By comparing near-field IR and confocal Raman images, we find that the stress-induced shifts of the longitudinal optical phonon-frequencies (LO) and the related shift of the phonon-polariton near-field resonance give rise to Raman and s-SNOM image contrasts, respectively. We apply single-frequency IR s-SNOM for nanoscale resolved imaging of local stress-fields and confocal Raman microscopy to obtain the complete spectral information about stress-induced shifts of the phonon frequencies at diffraction limited spatial resolution. The spatial extension of the local stress-field around the nanoindent agrees well between both techniques. Our results demonstrate that both methods ideally complement each other, allowing for the detailed analysis of stress-fields at e.g. material and grain boundaries, in Micro-Electro-Mechanical-Systems (MEMS), or in engineered nanostructures.

(color online): Confocal Raman microscopy and s-SNOM images of a locally stressed SiC sample. (a) Rayleigh intensity map. Four regions of interest are labeled A-D, (b, c) maps of the spectral position of the fitted TO and LO phonon lines as obtained by fitting a Lorentzian. Black triangles mark the contour of the indent. For comparison, s-SNOM amplitude maps of the identical indent are given in (d-f). (d) Shows the topography of the indent, (e) s-SNOM amplitude detected for ωIR = 924 cm−1 and (f) ωIR = 944 cm−1 laser frequencies reveal the characteristic amplitude contrasts around the indent. Note that in the near-field optical images the scratches exhibit a pronounced contrast, which can be explained by crystal damage caused by the mechanical polishing process [33].

(color online): s-SNOM hardware zoom into the right corner of the indent revealing a nanocrack emanating from the edge. (a) Topography and (b) s-SNOM amplitude at ωIR = 935 cm−1. The inset in (a) shows an additional hardware-zoom of the corner of the indent revealing a nanocrack in the topography data.